After decades of research, a genetic therapy for Huntington’s disease is being tested in clinical trials. Sponsored by Swiss pharmaceutical firm Roche and US-based Ionis Pharmaceuticals, this trial targets the gene that causes the disease. If the new treatment works, it could offer a way to halt progression of this genetic disease — an awful neurodegenerative disorder that attacks mainly the brain. Huntington’s is caused by a single gene mutation transmitted in a dominant fashion, so a child has a 50% chance of inheriting the condition if one of their parents carries a single copy of the defective gene.

Ferdinand, 42, lost his job last year. His father having died early, neither Ferdinand or his wife knew the disease was in the family. Photograph: Nick Garcia for the Observer.

As the small motorboat chugs to a halt, three travellers, wind-beaten from the three-hour journey along the Atrato river, step on to the muddy banks of Bellavista, an otherwise inaccessible town in the heart of the heavily forested north-west of Colombia. They swing their hessian bags – stuffed with bedsheets, dried beans and cuddly toys – to their shoulders and clamber up a dusty path. Tucked inside the bag of one of the travellers, neuropsychologist Sonia Moreno, is the reason they are here: a wad of unfinished, hand-drawn charts of family trees.